Direct injection technology is a way of directly injecting fuels into an engine with spark-ignition cylinders. Direct-injection engines have great fuel economy. They represent important development for future engines. The most important part for direct injection is the fuel-supplying system. A good fuel-supply system should satisfy as much as possible the combustion, performance and discharge requirements of the engines. The goal is to have direct injection engines that are affordable and easy to use.
Gasoline Direct Injection (GDI) is used in an increasing number of car engines. Most of the direct-injection systems used in car engines are common-rail fuel line injection systems. Except during the start-up process, the pressure in the common-rail fuel lines typically remains between 8 and 20 MPa. Currently, the method to build such pressure in the common-rail fuel lines relies on mechanical plunger pumps with electromagnetic controls. These pumps are driven by cams. When installing such pumps, the starter has to be redesigned. In addition, mechanical GDI high pressure pumps have several disadvantages as follows:
1) Unstable pressure in the fuel rail before engine starts. When not used for a long time, the pressure will decrease to under 1 MPa, causing problems in engine start and the subsequent transition process, and also causing the engine to emit pollutants.
2) Unstable pressure in the fuel rail, and the pressure varies significantly with different phases of cams.
3) Complicated working conditions in transitioning from complete stoppage of fuel supply to resupplying fuels. It is hard to maintain the same rail pressure while the fuel stops or during engine idle.
4) When under partial loads, fuel is repeatedly heated. The low pressure metal matric diaphragm (MMD) is adversely impacted by dual effects of temperature and alternating pressure.
5) There is a strong link between the computational logic for the amount of fuel needed by engines and the regulation of the high-pressure pump. This results in complicated control logic.
6) If the fuel rail has a limited capacity, the pressure fluctuation would be increased. If the fuel rail capacity is too large, a long process would be needed to establish the pressure before starting.
In sum, the above-described problems and dilemma exist in current GDI mechanic pumps. To completely overcome these problems, new approaches to alternative pump technology is needed. In comparison, electronic fuel pumps do not have the problems mentioned above. The advantages of electronic fuel pumps include: they can establish high pressure before engines start; they can increase fuel rail capacity without limitations or introduce buffers, therefor achieving constant pressure injection by minimizing fuel rail pressure fluctuations; they can more precisely supply fuel as needed; when fuel is not needed, it can completely stop working; the fuel pumps have little impact on fuel lines; and the fuel pumps are independent of the engines, making it easier to install, produce and service.
However, it is difficult for current electronic fuel pumps to establish fuel pressure that is over 8 MPa. The pressure established in rotary electronic fuel pumps is no more than 3 MPa. Theoretically, the pressure achievable by a plunger pump driven by a rotary motor is no different from that achievable by a mechanic pump. However, the efficiency is much lower for a rotary motor driven one, and it costs more than a mechanic pump. Current methods using linear motor to directly drive a plunger pump, instead of cams, result in low energy conversion efficiency and low time utilization efficiency. To achieve high pressure using these methods, the products would become bulky and costly.